For many uses, particularly in the area of biomaterials for medical applications, it is beneficial to modify the internal surfaces of tubing made of dielectric materials. This must ordinarily be accomplished in a procedure that occurs after the tubing itself has been manufactured. Treatment of a surface with a gas plasma is a versatile method for modifying the surface of a solid material. A major advantage of this approach is the capability of applying very thin, highly adherent, uniform coatings free of pinholes, without changing the composition and physical characteristics of the underlying material being coated. This can be achieved by means of a gas plasma that employs a polymerizing monomer or mixture of such monomers. Alternatively, gas plasmas may also be used to clean and crosslink naturally occurring surfaces of materials while not depositing polymerizates, such as by treatment with plasmas from gaseous compositions that do not contain polymerizing monomers. It is nevertheless a major obstacle to introduce a reactive gas or gas mixture into the lumen of a continuous length of tubing that is not itself naturally porous along its length. Even materials assumed to be naturally porous to some extent may not be amenable to treatment of their lumen surfaces with a gas plasma, if the treatment is to be accomplished in the context of a continuous process operating on a continuous length of material. For instance, gas plasma treatment of the inner surface of vascular grafts has heretofore been essentially limited to the processing of short lengths. The walls of vascular grafts, while thin and flexible, are surprisingly tight, such that a pressure difference of as little as one torr between the outside and inside of the vascular graft under the vacuum conditions employed in gas plasma treatment is sufficient to collapse the graft. Thus, many types of vascular grafts cannot be plasma-treated on their lumen surfaces in a continuous process operating on continuous lengths.
Efforts have been made by others to treat lumen surfaces of some types of tubing by means of gas plasmas. U.S. Pat. No. 4,632,842 discloses a method of coating the surfaces and pores of a porous tubular substrate with a polymerizing gas plasma to generate a material useful as a vascular graft. U.S. Pat. No. 4,687,482 discloses a vascular prosthesis having a hydrophobic polymeric layer deposited on its lumen surface such as by gas plasma treatment. U.S. Pat. No. 5,034,265 discloses a method of coating a woven or knit vascular prosthesis with a gas plasma by which a fluorinated hydrocarbon monomer is grafted to its surface. U.S. Pat. No. 5,244,654 discloses a method to treat the internal surface of a silicone rubber tube of short length with an oxidizing gas plasma to prepare a surface for subsequent derivatization with biocompatibility enhancing agents. U.S. Pat. No. 4,948,628 discloses an apparatus and method for treating the lumen surface of a small diameter tube by drawing a long length of it through a bore in a two-chamber housing having a gas pressure differential between the two chambers, delivering radiofrequency power to gases in the tube lumen while it is within the bore. U.S. Pat. No. 4,692,347 discloses a method of selectively coating the lumen surface of a long length of tubing by introducing a reactive gas at one end of its length, removing residual or unreacted gas from the other end by vacuum (which establishes a flow of gas through the lumen), and passing the tubing through a radiofrequency field wherein the gas mixture flowing in its lumen is excited to a plasma state. This latter method was capable of selectively coating the lumen surface of lengths of tubing as long as 10 to 15 meters.
None of these patents specifically disclose methods and apparatus for plasma-treating the lumen walls of continuous length tubing. In this respect, the above approaches appear to lack a key element for widespread commercial utility. The latter two disclosures indicate that long lengths may be plasma-treated within the tubing lumen. Nevertheless, they do not appear to provide a necessary control of monomer pressure and flow rate to achieve uniform plasma polymerizate coatings on tubing lumen surfaces.
A pressure drop occurs when gas flows through a lumen of a tube, and the magnitude of this pressure drop restricts the length of the tube that can be properly treated. In U.S. Pat. No. 4,692,347, silicone rubber tubing was used that had an internal diameter of 3.3 mm. From an M.S. thesis by Y. Matsuzawa, Continuous Plasma Polymerization for Preparation of Composite Reverse Osmosis Hollow Fiber Membranes and of Biomedical Plastic Tubing (see p. 64, FIG. 27), tubing such as this was found to have a pressure drop in excess of 700 mtorr per meter at an argon gas feed rate of 0.104 sccm (standard cubic centimeters per minute). Because of this pressure drop, the pressure of a reactive gas in the lumen must unfortunately be quite nonuniform along its length. For a polymerizing monomer, the nonuniform gas pressure leads to nonuniform deposition rates of plasma polymerizate on lumen walls along the length of the tubing lumen. Similarly, crosslinking or other surface modifications of lumen surfaces by nonpolymerizing gas plasmas will be nonuniform along the tubing length. Furthermore, some gases readily permeate through the walls of certain materials such as silicone rubber, such that the pressure of monomer vapor inside the lumen cannot be maintained in an optimum range when the outside of the tube is under a vacuum. In the specific case of silicone rubber, efforts to apply plasma coatings from silane or siloxane gaseous monomers following the approach of U.S. Pat. No. 4,692,347 will generally fail because of monomer losses through the tubing wall, and no such examples are given in that patent's description.
The need remains for an approach that can internally coat or modify the lumen surface of a tubing wherein the tubing is continuous, i.e., not limited to 10 meters, 15 meters, or even 100 meters in treatable length. Further, the need remains for a method that can treat the lumen surfaces of thin-walled tubing such as vascular grafts wherein the wall porosity is so low that minor pressure differences are sufficient to collapse the tubing in a gas plasma treatment operation. Further, a need exists for a method whereby nonporous or nearly nonporous tubing can be continuously coated in a commercially attractive manner that does not limit the length of the material to be processed. In addition, a need exists whereby the monomer feed rate to a polymerizing plasma zone can be controlled to a narrow range so as to lay down an essentially uniform thickness of plasma polymerizate on lumen surfaces.
According to Yasuda (U.S. Pat. No. 4,692,347), the energy input per unit mass of monomer is a significant parameter in determining the character of the polymeric coating obtained in a plasma polymerization. Thus, for example, where
W=power input in watts PA1 F=monomer flow rate in moles per second PA1 M=molecular weight of the monomer
a tight amorphous polymeric layer effective as a barrier layer was obtained by Yasuda where W/FM was equal to 10.sup.9 to 10.sup.10 Joules per kilogram (J/kg). A relatively permeable coating was obtained where W/FM was equal to 10.sup.7 to 10.sup.8 /kg. Thicknesses in the range of 100 to 500 angstroms were preferred. At thicknesses significantly greater than 1000 .ANG., the coatings had a tendency to suffer from stress cracking. By way of example, fluoropolymer coatings deposited from a glow discharge plasma of tetrafluoroethylene have been noted by Matzuwara (loc. cit.) to show cracks when the flow rate of the monomer was increased beyond the range of 0.115-0.331 cm.sup.3 (STP)/min in a glow discharge polymerization operated in the range of 6-14 watts in silicone rubber tubing at a residence time of 3-20 seconds. For many purposes, a barrier layer 100-500 .ANG. thick deposited by plasma polymerization on the lumen of a tubing is inadequate. The surface roughness of the lumen of a tubing may be too large to be adequately protected or sealed by such a thin deposit of plasma polymerizate. As an example, elastomeric tubing filled with an opacifying pigment, such as titanium dioxide in a white silicone rubber tubing, will tend to have a surface made rougher by the presence of pigment particles in the surface layer. Additionally, barrier layers deposited on the tubing lumen surfaces ideally should be able to withstand the insertion of connectors into the tubing without significant loss of integrity. Thicker barrier layers are needed, possessing thereby sufficient abrasion resistance to safely accommodate insertion of connectors. Such thicknesses do not appear possible within the confines of the method disclosed by Yasuda because of stress cracking.
An object of the invention therefore is to modify the lumen surface of a continuous length of nonporous or sparingly porous tubing by a gas plasma. By "continuous length" is meant a span of tubing up to kilometers in reach, that is not limited in extent by reason of accessibility of its lumen to reactive gas or gas mixture and establishment of a suitable gas flow for gas plasma development therein. Thus, the tubing may be limited in length to a kilometer or fraction thereof by reason of physical constraints of a plasma apparatus but not by reason of the method of the invention.
Another object of the invention is to modify the lumen surface of a continuous length of the tubing by deposition of a gas plasma polymerizate on the lumen wall out of a polymerizing gas plasma.
Another object of the invention is to deposit plasma polymerizate coatings on the lumen wall of the tubing wherein the coating thickness can be well in excess of 1000 .ANG. while being essentially free of stress cracking.
Another object of the invention is to supply monomer at a narrow range of feed rate to the plasma zone in a tubing lumen throughout the duration of movement of a continuous length of tubing through the plasma treatment zone.
Another object of the invention is to selectively modify the lumen surface of a continuous length of the tubing by a gas plasma, wherewith the external surface of the tubing remains essentially unmodified.
An additional object of the invention is to modify the lumen surface of a continuous length of tubing intended for medical applications such as for catheters, vascular grafts and feeding tubes.
Additional objects, advantages and novel features of the invention will be set forth in the description of the invention which follows, and in part will become apparent to those skilled in the art upon review of the following description or as may be learned through practice of the invention.